In recent years, there is an increasing need for electrophotographic color image forming devices such as a multifunctional printer, a facsimile machine and a printer. Color printing is required to reproduce color tones that are as high-resolution and sharp as photographs. Accordingly, there is a demand for color toners that can meet the request. Such toners are required to have various kinds of printing properties such as environmental stability (from the viewpoint of preventing a deterioration in image quality due to an environmental change in temperature, humidity, etc.), printing durability (from the viewpoint of reducing printing costs), and low-temperature fixability (from the viewpoint of reducing power consumption).
To meet the request, a spherical toner with a small particle diameter is suitable, which can achieve both excellent transferability and dot reproducibility. As the method for producing the toner, a polymerization method (a wet granulation method) has been proposed. In the case of a conventional pulverization method, especially in the case of producing a toner with a small particle diameter by the method, a low yield is obtained, and a lot of energy is consumed by pulverization. Meanwhile, in the case of the polymerization method, a high yield is obtained; energy consumption is low since a pulverization process is not necessary; and a spherical toner can be easily produced.
In the case of obtaining colored resin particles by the polymerization method, it is advantageous in that at the step of forming particles (the step of forming and polymerizing droplets in the case of the polymerization method, and the step of pulverization in the case of the pulverization method), spherical colored resin particles with a smaller particle diameter and a narrower particle size distribution than those produced by the conventional pulverization method, can be famed.
However, along with a further increase in the level of demand for high resolution and high quality, it has been pointed out that the polymerization toner has problems that must be solved.
In the case of producing a toner by the above-described polymerization method, there is a problem in that it is difficult to uniformly disperse a pigment (colorant) in the toner, and the image density of an image thus famed decreases. As a means to solve the problem, if the amount of the pigment added to the toner is increased so as to obtain sufficient image density, aggregation of the pigment occurs in the toner and causes the charge performance of the toner to be unstable, resulting in problems such as a void and toner scattering.
As a method to solve the problem, in Patent Document 1, it is disclosed that in the case of producing a toner by a wet granulation method, if a pigment subjected to a coupling treatment with a coupling agent is used, the dispersibility of the colorant is increased; the chargeability of the toner is stabilized; and an image with sufficient image density is obtained, preventing a void toner scattering.